1. Technical Field
The present disclosure relates to an apparatus for controlling an inverter.
2. Description of the Related Art
A medium voltage inverter refers to an inverter using an input power greater than 600 VRMS of line voltage, and its rated power capacity ranges from several hundreds of kW to several tens of MW. Medium voltage inverters are commonly used in fans, pumps, compressors, etc. Among such medium voltage inverters, a cascaded multi-level inverter is frequently used, of which output phase-voltages have three or more output voltage levels. The magnitude and the number of output voltage levels of a multi-level inverter are determined depending on the number of power cells thereof. Each of the power cells uses an isolated input voltage.
Typically, a medium voltage electric motor driven by a medium voltage inverter has very high inertia. Accordingly, when an error takes place in an input power or service interruption occurs, it takes long time for the electric motor to completely stop operating for restarting. In order to reduce the time period for restarting, a voltage may be applied according to a voltage/frequency ratio (V/f) during the rotation of the electric motor. However, this may result in a large inrush current to cause malfunction in an inverter or an electric motor.
For this reason, in order to reduce time period for restarting and avoid malfunction in an inverter or an electric motor, a voltage measuring device is employed.
An existing voltage measuring device measures an induced electromotive force of an electric motor in rotation under no control, to learn the voltage and speed information of the electric motor for restarting. When an inverter generates an output, however, the voltage measuring device measures the output from the inverter, and thus can no longer learn the information on the state of the electric motor.
An inverter does not control phase-currents of an electric motor as it is operated according to voltage/frequency. When the frequency of the voltage of an electric motor measured during free rotation is used as an output frequency of an inverter, the amplifier of the current of the electric motor is determined depending on the size of a load.
In the free rotation state of an electric motor, which is triggered as a protective operation against an error in an input power, if the size of a load is small, the amplitude of a generated current is small because the magnitude of the induced electromotive force and reduction in frequency are small, and the slip frequency after voltage for restarting is output is small. However, if the size of the load is large, the amplitude of the generated current is large because the magnitude of the induced electromotive force and the reduction in frequency are large, and the slip frequency after voltage for restarting is output is large.
As such, in the large load condition, the induced electromotive force is largely reduced and thus the magnitude of the available voltage for restarting is small. In addition, a larger current capacity is required due to a larger slip frequency.